Setting am/pm time on lubuntu

Welcome to AskUbuntu. Do you mind telling us where exactly do you want to see that time's format? If you wish to use it in a terminal you can drop date +%r which will return the time in the next format:

More information in the date's man page at: http://manpages.ubuntu.com/manpages/lucid/man1/date.1.html

If you wish to change panel's time format, take a look at this LUbuntu's install/tweak guide, specially on "Step #09" which reads the next:

Step #09: Customize The Clock (Show Date and Time AM/PM)

By default, Lubuntu does not show the date in the panel and only shows 24-hour time. This is annoying especially since you take the time to choose your timezone during the installation. If you're in the USA like me, you can have the Lubuntu panel display the proper time format with a little tweak.

Right click on the clock and select "Digital Clock Settings" and change the Clock Format and Tooltip Format's to the following:

Clock Format: %m/%d/%y %I:%M
Tooltip Format: %A %x

But if you wish to set the time format in a different place please let us know by editing your question, after which this answer can be edited or deleted.

Thank you and Good luck!